In an orthogonal frequency division multiple access (OFDMA) communications system, time-frequency resources of the system are shared among a plurality of mobile stations (MS). A base station (BS) assigns resources to MS using an assignment message, which is typically transmitted as part of a control channel. To minimize control channel overhead, it is known for the BS to make persistent assignments, wherein the assignment message is transmitted to the MS initially to indicate an assigned time-frequency resource, and then the BS uses the same time-frequency resource for subsequent transmissions to (or receptions from) the MS.
The subsequent transmissions (or receptions) may be hybrid automatic repeat request (H-ARQ) transmissions of the same packet, for subsequent transmissions of different packets, or for initial transmissions of a series of packets. For example, in a voice over internet protocol (VoIP) system, the BS make a persistent resource assignment to a particular MS consisting of a particular time-frequency resource with a period equal to 20 msec (equal to a vocoder period). Then, the BS may use this resource for a first H-ARQ transmission of each VoIP packet. Subsequent transmissions, if necessary, must be signaled.
The initially assigned time-frequency resource may be maintained by the BS for the MS until a timer elapses, a VoIP talk-spurt is completed, a VoIP call is completed, a certain number of negative acknowledgements is detected by the base station, until the resource is explicitly or implicitly de-assigned by the base station, or so on. When one of these events occurs (i.e., when the persistent assignment expires), there may be a hole in the set of time-frequency resources of the communications system, with the released previously allocated time-frequency resource(s) being referred to as a resource hole. The BS may assign the released previously allocated time-frequency resource(s) (i.e., the resource hole) to a new MS if the number of resources required by the new MS is equal to or less than the number of resources in the resource hole. Unfortunately, due to different packet sizes, modulation and coding characteristics, bandwidth requirements, and so forth, the number of resources required by the new MS may be different than the number of resources in the resource hole. Thus, there is a need for efficiently allocating time-frequency resources resulting from expired persistent assignments.